The field of invention relates generally to electronic circuit design; and more specifically, to a method and apparatus for a resetable memory and a design approach for same.
FIG. 1 shows a memory unit 101. The memory unit 101 may be viewed as having a plurality of storage cells (or simply, xe2x80x9ccellsxe2x80x9d). Associated with each cell is a unique address that provides access to the location of a particular storage cell. Each storage cell has the capacity to store xe2x80x9cnxe2x80x9d bits (where n is an integer greater than or equal to one). The n bits may be collectively referred to as a word of data.
Often, a memory unit 101 is written to by: 1) providing a word of data (i.e., xe2x80x9cnxe2x80x9d bits of data as seen in FIG. 1) to be written into the memory unit 101 on a data bus (such as input data bus 103 observed in FIG. 1); 2) providing an address (e.g., on address bus 102) that defines which storage cell will store the word of data; and 3) presenting a signal to the memory unit that effectively indicates the word of data on the data bus is to be written into the memory unit (such activating the write enable (WE) control line 104 of FIG. 1).
Alternatively, the write could be time based as in a shift register, or it could be a mix of address and time based write.
Often, a memory unit 101 is read from by: 1) providing an address (e.g., on address bus 102) that defines which cell a word of data will be read from; and 2) presenting a signal to the memory unit that effectively indicates a word of data is to be read from the memory unit (such as activating the read enable (RE) control line 130 of FIG. 1). The word of data is presented at the data output bus 106. In embodiments alternative to that observed in FIG. 1, the data in and data out buses 103, 106 may be combined to form a bi-directional bus. Also, in various alternate embodiments, one of the enable lines 104, 130 may be eliminated so that a single line is used to toggle the memory unit between being in a writable state and being in a readable state. Alternatively, it could be time based as in a shift register, or a mix of address and time based read. A commercial example that uses a mix of address and time based read/write is the Xilinx Virtex SRL primitive that is written into like a shift register and is read from like a RAM.
The memory unit 101 of FIG. 1 can be used to implement a number of storage related devices such as a random access memory (RAM), a first-in-first-out (FIFO) queue (e.g., by appropriately controlling the address values of the memory unit 101 such that a FIFO queue is emulated with the memory unit 101), a content addressable memory (CAM), a shift register, etc. A problem with memory units (as they are offered to designers who wish to employ them in their circuit designs), however, is that they do not have a reset function. A reset function effectively xe2x80x9cclearsxe2x80x9d the memory unit""s cell word values to some xe2x80x9cresetxe2x80x9d value (e.g., a n wide value of xe2x80x9c0xe2x80x9d); and, often, the integration of circuitry for resetting the cell word values of the memory unit 101 is too expensive and/or complicated to implement. For example, according to one approach, in order to implement a resetable memory, each n wide storage cell is implemented with resetable flip-flops that are individually accessed via complicated multiplexing and control circuitry. Here, the use of resetable flip-flops to implement each n wide storage cell (as well as the complicated multiplexing and control circuitry) can result in a resetable memory unit having noticeably slower performance (and that consumes more silicon surface area) than a memory unit that does not have resetable storage cells.
An apparatus is described that includes a memory without reset capability having a data output coupled to a first input of a first multiplexer. A second input of the first multiplexer has a reset value input. A channel select for the first multiplexer is coupled to a resetable storage cell output that indicates whether a storage cell within the memory without reset capability has been written to after a reset or has not been written to after a reset.